For manufacturing electronic devices to have a small size and a light weight, demands for secondary batteries which have a light weight and a high energy density have increased. Further, secondary batteries having a fast charging/discharging speed and high safety are needed in electric vehicles or hybrid vehicles.
A secondary battery draws out chemical energy of a positive electrode active material and a negative electrode active material into the form of electrical energy by a chemical reaction via an electrolyte, and a non-aqueous lithium secondary battery is a typical secondary battery in the practical step.
Generally, a lithium-containing metal composite oxide having a spinel structure such as a lithium cobalt composite oxide is used as a positive electrode active material in the lithium secondary battery, and a carbon material, which is represented by graphite having a multilayer structure capable of intercalation of lithium metal, is used as a negative electrode active material. In recent years, a hard carbon-based material that may replace graphite due to necessary characteristics such as charging/discharging characteristic has been on the spotlight. However, a battery including a carbon material as a negative electrode active material has a capacity of about 360 mAh/g, which is close to a theoretical capacity (370 mAh/g) and cannot expect a significant capacity increase, and thus a negative electrode active material having a capacity that is higher than a capacity of carbon is essential to manufacture a high capacity battery as an energy source of a high function portable electronic device or an electric vehicle in the future. Recently, hard carbon and complexation thereof has been noticed, and Patent Documents 1 and 2 disclose attempting complexation with a material that is referred to as an alloy-based material, such as silicon. However, Patent Document 1 does not correspond with the necessary characteristics in terms of fine dispersion of silicon and adhesion with a carbon material that are important factors related to the silicon-based material, and neither does Patent Document 2 in terms of yield and initial efficiency of silicon.
In this regard, studies for practical use of an alloy-based negative electrode material including elements such as silicon or tin have been conducted. A metal element such as silicon or tin may form a metal lithium within a broad temperature range, which allows electrochemical intercalation of lithium and deintercalation of lithium ions. Also, the metal element may allow charging/discharging at a significantly large capacity, compared with that of a carbon material (where a theoretical discharge capacity of silicon is about 4200 mAh/g).
However, an alloy material including elements such as silicon or tin may cause a large volume change during intercalation and deintercalation of lithium, which may accordingly generate pulverization or fine grinding of the active material and defects in a current collector by detachment of a negative electrode layer, and thus cycle characteristics may degrade as a result.
In order to resolve the problems, improvement in various aspects such as a negative electrode material itself, a binder, and a current collector have been reviewed, and, particularly, realizing a fine structure of an active material or reducing capacity deterioration (reduction of volume change during charging/discharging) by complexation have been suggested in terms of the negative electrode material, and improvements in this regard have been achieved to a certain degree.
For example, Patent Document 3 discloses a negative electrode active material including an amorphous material that is represented by SiMx (where 0≤x≤2 and M is at least one element selected from Ti, Ni, Cu, Co, and Fe), and Patent Document 4 discloses a negative electrode active material using a Ti—Si alloy. However, a transition metal (M) such as Ti has a high density and a heavy weight, which limit the application of the material in an electronic device that is in the trend of having a light weight.
Also, Patent Document 5 discloses a conductive silicon complex that is carbon coated at a high temperature by using a silicon oxide (SiO) as a raw material. Although cycle characteristics of a lithium secondary battery prepared by using the silicon complex as a negative electrode material improve significantly, the initial efficiency and high rate characteristics of the lithium secondary battery are not sufficient.